1. Field of the Invention
The present invention relates to a structure of a display device utilizing a liquid crystal as a display medium (called liquid crystal display device or LCD). More particularly, the invention is concerned with a technology on an electrode structure for applying a predetermined voltage to the liquid crystal.
2. Description of the Related Art
CRTs are most popular as conventional displays. However, the CRTs are large in volume, weight and power consumption, and particularly not suited as a display unit with a large screen. Under such a situation, attention has recently been drawn to liquid crystal display devices (hereinafter referred to as xe2x80x9cliquid crystal panelxe2x80x9d) that are advantageous over the CRTs with respect to light-weight, low power consumption and large display screen.
Although the drive methods differ depending upon a liquid crystal kind used in a liquid crystal panel, there is known a drive method using liquid crystal birefringence. This is a drive method utilizing a nature that liquid crystal molecules have different dielectric constants between a longer axial direction and a shorter axial direction thereby controlling light polarization, transmission amount and further light scattering amount. The usable liquid crystal materials include a nematic liquid crystal, ferroelectric liquid crystal, anti-ferroelectric liquid crystal and so on.
In particular, there is a recent development for liquid crystal panels formed with a plurality of thin film transistors (TFT) using a thin film semiconductor on a glass substrate wherein circuits are formed by these TFTs. Such liquid crystal panels are particularly called an active matrix liquid crystal panel.
The active matrix liquid crystal panels are roughly divided in display scheme as a transparent type and a reflective type. In the reflective liquid crystal panel, the light transmitted through a liquid crystal layer is reflected by pixel electrodes provided on pixels. The reflection light (light containing image information) is received by a utilizer""s eye thus making it possible to view images.
The reflective liquid crystal panel (hereinafter referred to as xe2x80x9creflective LCDxe2x80x9d) has an advantage that the effective pixel area is broad as compared to the transmission liquid crystal panel. This is due to no limitation in the opening ratio, which is different from the transmission liquid crystal panel. Consequently, the reflective type is capable of displaying with brightness higher than that of the transmission type.
However, it is that the reflective LCD undergoes higher light loss as compared to the transmission LCD. Consequently, the greatest problem with the reflective LCD hinges on how much incoming light is effectively utilized. That is, it is an essentially required factor to utilize light as much as possible and reduce light loss, in order to provide brighter display.
For these reasons, it is preferred to enhance the pixel electrode reflectivity as high as possible. This is because if the reflectivity is low the utilization efficiency of light considerably decreases thereby darkening the whole display. Meanwhile, even if the backlight light amount is increased to increase display brightness, countermeasures will be required against problems of increasing power consumption and heat generation thus resulting in cost increase.
In order to reduce the light loss, the pixel electrodes for a reflective LCD use an aluminum based material (pure aluminum, aluminum alloy, or aluminum containing impurities) that is high in reflectivity. Incidentally, although silver electrodes are characterized by further high reflectivity, there is difficulty in formability. In many cases, the aluminum based material is utilized that is comparatively easy to handle.
However, it was revealed from a study by the present applicant that there is great reduction in reflectivity in a state that a material with high refractive index like an orientation film is formed on an electrode surface as compared to a case that an electrode formed by an aluminum based material is directly radiated by light.
In this manner, even if a high reflective material is selected for a pixel electrode, the reflectivity is impaired due to other factors. Further devising is desired toward increase in reflectivity.
The present invention provides a means for solving such a problem. That is, a technology is disclosed to improve the light reflection efficiency on the pixel electrode and hence improve the utilization efficiency of incoming light to a reflective LCD.
It is an object to realize a liquid crystal panel which is low in cost, bright and has low power consumption, and further realize an electronic appliance mounted with a liquid crystal panel as a display unit.
The present invention is characterized in that, in a reflective liquid crystal panel, reflective light is amplified by forming an enhanced reflective film by a dielectric multi-layered film on the pixel electrode thereby effectively utilizing the light.
Incidentally, the dielectric mirror is known as a technology utilizing an enhanced reflection effect on the dielectric multi-layered film. However, the dielectric mirror, in most cases, is formed by overlying eight or more layers. The directly connected capacitance that is formed by the dielectric mirror is extremely small.
In an actual liquid crystal panel, there is series connection of the dielectric multi-layered film capacitance (Cd) and the resultant capacitance (C1c) of a liquid crystal and orientation film. Consequently, if the total capacitance is Ctotal, the following relation stands:
1/Ctotal=1/Cd+1/C1c
Meanwhile, if the voltage applied to the resultant capacitance is Vtotal, the voltage applied to the liquid crystal and orientation film is V1c, and the voltage applied to the dielectric multi-layered film is Vd, the following expression stands:
CtotalVtotal=CdVd=C1c V1c
By deforming this expression to determine a voltage applied to the liquid crystal, the following is obtained:
V1c=CdVtotal/(C1c+Cd)
That is, where the capacitance formed by the dielectric multi-layered film is low, there arises a problem that the voltage applied to the liquid crystal decreases.
However, the present invention takes it into consideration use of a dielectric multi-layered film as an electrode, and is reduced in the number of overlying layers, i.e. two layers or four layers. Because the capacitance of the dielectric multi-layered film is large as compared to the capacitance of the liquid crystal, an advantage is obtainable such that the present invention is free from the aforementioned problem that the voltage applied to the liquid crystal decreases.
Meanwhile, where the liquid crystal display device is driven in active matrix scheme, if a liquid crystal cell having no dielectric multi-layered film has a peak voltage VLC.peak and TFT switching element withstand voltage Vmax, then in order to drive a liquid crystal cell having a dielectric multi-layered film within TFT withstand voltage, the dielectric multi-layered film must be controlled in the number of layers and film thickness to meet VLC.peakxe2x89xa6Cd Vmax/(C1c+Cd).
Here, the peak voltage (VLC.peak) means a voltage for giving a normally black mode brightness of 100% and a normally white mode brightness of 0% provided that a maximum brightness is taken 100% and a minimum brightness 0% on a V-T curve representative of a threshold characteristic of a liquid crystal cell having no dielectric multi-layered film, i.e., applied voltage to the liquid crystal cell (V) and liquid crystal cell brightness (T).